Communication system



Jan. 30, 1951 R. A. HElslNG 2,539,623

COMMUNICATION SYSTEM Filed Feb. 12, 1947 2 Sheets-Sheet l L* TERM '-H DECODER EQU/R A 7' TORNEY Jan. 30, 1951 R. A. HElslNG COMMUNICATION SYSTEM 2 Sheets-Sheet 2 /NVEN 70A A. A. HE/s/NG BV @52M ATTUQNEV n. 9 l om. l b. e F d m ew 3 n n .F E NQW QQ N ...El

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Patenied Jan. 30, 1951 COMMUNICATION SYSTEM Raymond A. Helsing, Summit. N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.. a corporation o! New York Application February 12, 1947, Serial No. 727,973

9 Claims. (Cl. 332-11) Ihis invention relates to signaling or communication systems for the transmission oi' complex wave forms oi' the type present in speech, music, telegraph, picture and like signals by the process or method sometimes known as pulse code modulation.

In communication systems utilizing this type oi transmission, a speech wave or other signal to be transmitted is sampled periodically to ascertain its instantaneous amplitude. The measured instantaneous amplitude is expressed by pulse codes analogous to telegraph codes.

One code which conveniently may beemployed in pulse code transmission involves permutations oi a ilxed number oi code elements each oi which may have any one oi several conditions or values. An advantageous code oi this type is the so-called binary code in which each oi the fixed number oi code elements may have either oi two values. One advantageous way o! representing these values is to represent one by a pulse sometimes referred to as an on pulse-and the other by the absence of a pulse sometimes referred to as an oil pulse. Alternatively, one value may be represented by a positive pulse and the other by a negative pulse. The total number of permutations obtainable with the binary code is proportional to 2 where n is the number oi code elements emp10yed Because the total number oi different amplitudes which may be represented by such a code of a fixed number of elements is limited, it is found desirable to divide the continuous range of amplitude values of which the transmitted signal is capable into a ilxed *number of constituent ranges which together encompass the total range. Each of these smaller or constituent amplitude ranges may then be treated asv i! it were a single amplitude instead of a range and is represented by an individualone of the permutations oi' the code. In the use of this method of code transmission the instantaneous' time ot sampling in the form of a binary code group o! a xed number of code element voltages, means for transmitting the several voltages either simultaneously or successively to a receiver and means for decoding the transmitted code groups to recover the initial complex wave form. Such systems involve careiul synchronization oi the transmitter and the receiver as well as synchronization between the sampling and coding means which must operate in accurately timed relationship. In many cases the coder operates only intermittently, and in some instances must be reset between samples to prepare it for coding the next sample amplitude.

In other instances it is desirable, through application oi well-known multiplex processes, to utilize a single transmission channel for the Purpose of transmitting several complex wave forms. It has been usual in such cases to syn- Vclironize the transmission of a code group with the operation of the sampling and coding circuits required to produce that code group. Thus it has been necessary to switch a common sampler and coder from one signal input to another, utilizing a single sampler and a single coder to provide representative code groups for each of the signal channels therein. Such operation imposes severe requirements on the sampling and coding equipments which must complete their operation in the time allotted to a single channel.

It is an object oi the invention to provide improved and simplied equipment for expressing the instantaneous amplitude of a complex wave form as a binary code group of voltages.

It is a further object oi the present invention to provide a code modulation communication system in which operation oi' the coding means may be substantially continuous and independent of that of the transmitting and receiving equipment.

It is a still further object of the invention to provide a coder employing feedback to insure accurate representation of the complex wave to be transmitted.

In accordance with the above, the invention relates in one aspect to a communication system comprising a coder adapted for continuously representing the amplitude of a complex wave in binary code form and means for interrupting the operation of the coder momentarily to permit transmission of the code group present at that time to a remote station.

In another aspect the invention relates to a coder comprising a binary counter responsive to the instantaneous amplitude of a complex wave to be transmitted to produce code groups of voltages representative of the amplitude oi' that complex wave.

In another aspect the invention relates to a communication system for transmitting complex waves du which a code representing the complex wave is`\generated, the complex wave is regenerated from this code, the regenerated wave is compared with the original wave and iinally the code is corrected in accordance with the comparison.

In other aspects the invention relates to a reversible binary counter and to the use of such a counter in the above-mentioned coder and to control means responsive to the amplitude of a complex wave to be transmitted for controlling the operation of such a counter.

'I'he above and other objects and features of the invention will be better understood from a consideration of the accompanying detailed description of a specific embodiment taken in con- Junction with the drawings in which:

Fig. 1 is a block diagram of a communication system in accordance with the invention; and

Fig. 2 is a circuit schematic of the transmitting equipment shown in block form in Fig. 1.

Referring to Fig. 1 which shows an illustrative embodiment of the invention in block form, a complex wave form to be transmitted, shown here as derived from a microphone I0, is applied to terminal equipment I2. It will be understood that the system of the invention may be employed with equal benefit for the transmission of complex waves from other sources, as for example those from telegraphic, photographic or vibration-actuated devices, and those from television video channels, facsimile transmitters or the like.

Terminal equipment I2 may contain ampliers, regulators, impedance transformers, etc. as necessary to convert the output of microphone I or any other source to a suitable power level. The signal appearing at the output of terminal equipment I2 comprising the complex wave form to be transmitted is superimposed upon a direct current bias voltage such that the signal will always be of the same polarity and is applied to apparatus for .expressing the instantaneous amplitude of the complex wave form as a binary code group of signals. This apparatus includes a counter I4, an oscillator I6 adapted to drive the counter, a comparator I8 and an electronic switch 20 arranged to control the operation of counter I4.

Counter I4, which is arrangedto measure in arbitrary units the instantaneous amplitude of the complex Wave. to be transmitted, comprises a multistage binary counting circuit. Each stage of this circuit has two conditions of stability, only one of which may exist at a time. The counter is formed by connecting these stages together in such a way that any stage is switched from one of its conditions of stability to the other only when the next preceding stage has been switched from one condition of stability to the other by one impulse and subsequently switched back tothe original condition of stability by a second impulse. In other words two switching operations of any stage are required to produce a single switching operation of the following stage. Thus each stage may be assigned to represent one of the denominational orders or positions of a binary number, the two conditions of stability of the stage corresponding respectively to the zero and the one of that order of the binary number. Then, if means are provided for each stage to produce a signal voltage indicative of the condition of stability occupied by that stage, a binary code-'representation of the number of impulses applied to the input or first stage of the counting circuit may be obtained.

Counter I4 is controlled by comparator I8 acting through electronic switch 28. Comparator I8 effectively controls the application oi impulses generated in oscillator I6 to counter I4 in such fashion that the number of impulses applied is proportional to the instantaneous amplitude of the complex wave to be transmitted. For this purpose means are included in counter I4 for producing a voltage the amplitude of which is representative of the number of impulses which have been applied to and counted by the counter. This lvoltage is compared in comparator I8 with a voltage proportional to the instantaneous amplitude of a complex wave to be transmitted. If the voltage representing the amplitude of the complex wave is larger than that representing the number of impulses counted by counter I4, electronic switch 20 completes a circuit from oscillator I6 to the counter causing it to operate until the two voltages applied to comparator I8 are equal.

Since the instantaneous amplitude of the complex wave to be transmitted may either increase or decrease, means are incorporated in the counter to permit reversal of the counting direction depending upon whether the voltage representing the complex wave or that representing the output of the counter is the larger. This reversing means is controlled by another section of electronic switch 28 in response to the output of comparator lIll and conditions counter I4 to count upwardly as the instantaneous amplitude of the complex wave increases and to reverse its counting direction and count down-- wardly as the instantaneous amplitude of a complex wave decreases.

vAs pointed out above, counter I4 provides an individual output signal for each stage and each output signal may have either of two values indicating which of the two conditions of stability is occupied by the stage of the counter from which it is derived. The several output signals taken together form a binary code-representation of the number of impulses counted andthus of the instantaneous amplitude of the complex wave applied through terminalv equipment I2. These output signals are applied individually to a radio transmitter 22 capable of producing a different subchannel carrier frequency for each of the counter output signals, one carrier being controlled by each of the counter output voltages. At predetermined instants, fixed in the case of this illustrative embodiment by timing oscillator 24, a gating or timing system 2S interrupts the action of the coding system and renders the transmitter operative to radiate simultaneously all subchannel carriers which have been enabled.

At the receiving end of the system, the radiated signals are received by radio receiver 28 and the demodulated output of that receiver is applied to a decoder 30. This decoder operates insf'e`sponse to the transmitted binary code groups to reconstruct the complex wave form represented thereby and its output is applied to terminal equipment 32 which comprises the necessary ampliiiers, lters and impedance changing devices required to produce an output suitable for asaaeas application to headphones Il or -any other output device which it may be desired to employ.

Circuit details of illustrative transmitting equipment in accordance with the invention are given in Fig. 2 to which reference is now made. The complex wave signal appearing at the output of terminal equipment I2 in Fig. 1 is applied through terminal 36 and potentiometer 50 to a comparator indicated at Il and comprising vacuum tubes 38 and 40 together with plate and bias batteries 42 and M, respectively, and load resisters 46 and 4l. Potentiometer 5I is connected to ground through a bias battery Il by means of which a direct current bias boltage is added to the complex wave signal at the output of the terminal equipment. 'I'his bias voltage has an amplitude which is at least equal to one-half the maximum peak-to-peak amplitude to be accepted by the coding circuits. Thus all signals applied to the coding circuits will be of one polarity. Assuming for example and as shown that the bias voltage is positive, and is equal to onehalf the peak-to-peak signal accepted by the coding circuits, then the maximum signal amplitude applied to the grid of comparator tube ll will be equal to the full amplitude which can be accommodated whenever the complex wave signal reaches its peak positive value. On the other hand. the signal applied to tube 38 ot the comparator will be substantally zero when the complex wave signal reaches its peak negative amplitude. Accordingly, tube 3l is never cut of! and. will operate at the approximate center of its range when no complex wave signal is applied (or when the complex wave amplitude is aero). 'I'he significance ot this condition will appear below.

The signal voltage appearing across potentiometer Il is applied to the control grid of vacuum tube 3l as stated above while a potential derived .from counter- Il and appearing across resistor 52 is applied to the control grid of vacuum tube 4I. The Vpotential applied to the comparator from amounts to 23 or 8. Similarly the use of five counting stages results in an increase in the number of possible permutations to 25 or 32. The nstage counter forming a portion of the coder of Fig. 2 accordingly permits expression of 2n permutation values.

Each of the counting stages 54, 56 and 58 comprises a pair of vacuum tubes 60 and 62, 64 and 66, and 6l and 10 respectively. Considering the first of these identical counting stages, im pulses appearing at terminal 12 are applied through capacitors 'I4 and 16 to the control grids of vacuum tubes 60 and 62, respectively. The cathodes of tubes l0 and 62 are connected together and to ground while the plates thereof are connected respectively through inductor 18 and load resistor 80 in series and inductor 82 and load resistor M in series to the positive terminal of battery 86, the negative terminal of which is grounded. The plate of tube Bil is connected through tapped resistor 88 to the negative terminal of bias battery 90, the positive terminal oi' which is grounded while the plate of tube i2 is connected through tapped resistor 92 to the negative terminal of bias battery 90. The control grids of tubes 00 and 62 are connected respectively to the taps of resistors 92 and t8. Thus a portion of the potential appearing at the plate of tube 6l is applied to the control grid of tube 62 and similarly a portion of the potential appearing at the plate of tube 62 is applied to the control grid of tube III.

The circuit arrangement including vacuum tubes .t and 62 and the associated circuit components and sources of potential is stable with current flowing in either one but not both of the tubes. Thus if it be assumed initially that curthe counter is derived therefrom in a manner I which will be described hereinafter and is proportional in amplitude to that represented by the y binary code group appearing at the output of the counter. Comparator Il controls the operation of counter Il through electronic switching circuits to determine when and in which direction the counter shall operate.

Counter Il comprises an impulse counting eircuit and certain auxiliary circuits permitting r'eversal oi' the counting direction thereof. The counting circuit comprises a plurality of stages each of which may include a switching circuit of the type sometimes referred to as a "nip-nop" circuit and having two mutually exclusive conditions of stability. Three such counting stages ll, 56 and 5l are shown in Fig. 2 representing the rst, second and nth stages of an n-stage counter. The dotted connections in Fig. 2 indicate the location of additional intermediate stages which are identical to stage 56 as shown. It should be understood that the number of counting stages employed depends in any particular instance, upon the range of amplitude desired to be accommodated by the transmission system and upon the required fidelity of transmission. If, for example, three stages are employed each having two conditions o! stability,

rent is ilowing through tube 62, tube et is cut oil. and the operation of the circuit may be considered as follows. When current is ilowing through tube 62 the plate potential thereof will be lower than that of tube because of potential drop in resistor M while no potentialappears across resistor l0 as there is no current in tube il. Since the grid of tube 60 has a connection to the plate of tube 62 through resistor s2, this grid is maintained suiliciently negative to prevent current iiow through tube t0. Since no current is ilowing in tube 6I the plate potential thereof is high and this potential applied to the grid ot tube 82 through resistor 88 maintains the potential of the grid of tube 62 at a suitable point to permit current ilow through that tube. If now a negative impulse is applied through terminal 12 and. capacitors 'Il and 16 to the control grids of vacuum tubes il and 62 the following action occurs. Since tube 80 is already cut oil the negathe possible number oi unique permutations.

tive impulse applied to the grid thereof has no direct eect. On the other hand the negative impulse applied to the control grid of tube B2 tends to cut of! the plate current in that tube and correspondingly increase the plate potential thereof. 'I'he diminution of current in tube 62 causes the potential drop across resistor 84 to disappear and allow the potential at the plate of tube $2 to approach that o! the plate of tube 60 and in addition the falling current in tube 62 causes inductance coil 82 in the plate circuit to produce an additional potential change, acting to raise the potential of the plate of tube 62 well above that of the plate of tube 60. Then due to the connection between the plate of tube 82 and the grid of tube 80, the potential of the grid of tube SII is increased sumciently to start current ilowinginthlstube. Astheplatecurrentbe- 7 gins to ow in tube 60. a reduction of its plate potential results. reducing further the voltage on the grid of tube 62 to maintainthat tube cut off. The current flow is thus switched ,from tube 62 to tube 60 and the circuitis shifted from one of its conditions of stability in which tube 62 conducts while tube 60 is cut oif to the other in which the reverse situation obtains.

It a second negative impulse is applied through capacitors 14 and 16 to the control grids of the two tubes a similar process occurs. Plate current through tube 60 is cut oil. The energy stored in inductor 18 is suillcient to cause initiation of plate current flow in tube 62 returning the ilip-ilop circuit to the initially assumed condition of stability. Thus it will be seen that i'or each pair oi negative impulses applied to counting stage 54 the ip-ilop circuit shifts from one condition of stability to the other and then shifts back to its original condition completing a counting cycle.

For purposes .of discussion, it is convenient to designate the two conditions of stability of such tlip-'ilop circuits as the o and on states, calling the tube which is conducting in the first of these states the oil ytube andthat which is conducting in the second the on tube. If vit is assumed that when current ilows in tube 32, counter stage 54 is in the oi state, tube 62 is the oil tube and tube 50 is the on tube. This convention will be followed hereinafter in referring to the tubes of the counter circuit.

The circuit arrangement of counting stage 56 is identical to that of counting stage 54, positive potential from battery 86 being applied to the plates of vacuum tubes 64 and 66 through load circuits including respectively inductor 94, re sistor 06 and inductor 90, resistor |00. Likewise the plate of tube 56 is connected through tapped resistor |02 to the negative terminal of bias battery 90 and the plate of tube 64 is connected through tapped resistor |04 to the same point. The grids of vacuum tubes 64 and 66 respectively are connected to the taps on resistors |02 and |04 and impulses may be applied to these grids through capacitors ||1 and ||9 which correspond to capacitors 14 and 16 of counting stage 54. As in the case of the counting stage described above, counting stage 56 has two conditions of stability and the current ilow is shifted from one of the tubes to the other by the application of a negative impulse through capacitors |1 and I I9 to the grids of the two tubes. As in counting stage' 54 two negative impulses must be applied successively to the grids to cause one complete counting cycle. Conforming to the convention chosen above, tube 66 of the counting stage 56 'will be designated the 01T tube and tube 64 the on tube.

The interstage connections which control the direction of counting will now be considered. In Fig. 2 the plate of on" tube 60 of counter stage 54 is connected through capacitor |06 to the control grid of vacuum tube |08 while the plate of olf tube 62 is similarly connected through capacitor to the control grid of vacuum tube ||2. The plates of vacuum tubes |08 and ||2 are connected together and through resistor ||4 to the positive terminal of battery |6, the negative terminal of which is grounded. The grid of 4tube |08 is connected through series resistor ||8 to the junction of voltage divider resistors |20 .and |22 which are connected in series between the negative terminal of battery |24 and control 8 is connected through resistor |28 to the junction of resistors |30 and |32 connected in series between the negative terminal of battery |24 and control lead |34.

The negative voltage from bias battery |24 is made suicient to maintain both interstage tubes |08 and ||2 cut olf in the presence of the largest possible potential difference between control leads |26 and |34. It, however, one of the control leads. |26 for example, is at a higher potential than the other control lead |84, tube |08 will be biased just below cut-off by the joint action of the bias voltage from battery |24 and that on control lead |26 while tube ||2 will be driven far below cut-oil.

Let it be assumed that control lead |26 is positive with respect to control `lead |84 and that interstage tube |08 isoperating just below cut-oil with tube I2 far beyond cut-oi and that counting stages 54 and 56 are in the off condition with oiT tubes 62 and 66 conducting and on tubes 60 and 64 cut oil. If a negative impulseis applied through terminal 12 and capacitors 14 and 16 respectively to the control grids of tubes 60 and 62. current ilow will shift from off tube 62 to on tube 60 as has been explained above. Thus the plate voltage of olf tube 62 increases while that of on tube 60 decreases.

Since interstage tube |12 is biased far below cut-off, the increase of voltage at the plate of oil" tube 52 has no effect on the operation of tube ||2. Similarly, the drop in plate voltage of on tube occasioned by the initiation of current ow therethrough has no effect on the operation oi' tube |08 which is already cut off as stated above.

lf a second impulse is now applied to the grids of vacuum tubes 60 and 62 of counting stage 54, this stage is returned to its initially assumed off condition with current ow shifted back from on tube 60 to off tube 62. As oil tube b2 again becomes conductive its plate voltage drops but this voltage drop, applied to the grid of interstage tube ||2 has no eect because this tube is already biased far beyond cut-off.

On the other hand, the increase in plate voltage brought about by the return 0f on tube 60 to the cut-oli condition and applied through capacitor |06 to interstage tube |08 is suilicient to cause the now of plate current through this tube which, it will be recalled, was assumed to be biased just below cut-olf by the potential across voitage divider resistor |20. The increase in plate current in tube |06 causes a corresponding drop in plate voltage and thus in the voltage applied through capacitors I |1 and ||9 to the grids of vacuum tubes 64 and 66, respectively, of counter stage 56. This drop in plate voltage corresponds to a negative pulse and is effective to shift the flow of current in counter stage 56 from oil tube 66 to on tube 64.

Thus it will be seen that two negative impulses must be applied at terminal 12 to produce one shift in current ow in counter stage 56 and that by the same analysis four negative impulses must be applied to terminal 12 to produce a full counting cycle of counting stage 56, shifting the flow of current from the off" tube 66 to the on tube 64 upon the second impulse and back to the oif" tube 66 upon the fourth count.

As shown in Fig. 2 the plates of off tubes 52, 66 and 10 of the first, second and nth counter stages 54, 56 and 58 are connected respectively to counter output terminals |86, |38

lead |25. Similarly the control grid of tube ||2 75 and |40. In the operation of the 'counter in described above. it is seen that upon the application of a single negative impulse -under the initial conditions assumed above with current flowing in oif" tubes t2, it and 1li, the voltage at terminal I is increased to a value greater than that initially present. Thus increased output voltage is present at terminal ill while that at terminal III remains at the initial value. ThuLthe voltages on terminals Ill and I in that order may represent the binary number 01. corresponding to a count of one impulse. Upon receipt of a second negative impulse counter stage Il returns to its initial condition completing a` full cycle with the current flow again throimh oil tube t2, a'nd applying a single negative impulse to counter stage Wto shift the iiow of current therein from 03" tube 88 to on" tube Thus the voltage at terminal |36 is reduced yand an increase in output voltage appears at terminal III. In this instance the binary number obtained on terminals |38, |36 in that order is l corresponding to a count of two.

A third impulse applied at terminal 12 againswitches counter stage 5l to the on condition (as contrasted with the initially occupied oil'" condition) and the ilow of current is again switched from olf tube i2 to on" tube 60. Accordingly, the plate voltage of o tube t2 increases and the voltage appearing at output terminal llt is increased signifying operation of the stage. Since the third impulse causes only one-half cycle of operation of counter stage 54 no negative impulse is applied to counter stage It and it remains in the on condition to which it was switched by the second impulse. Thus after three impulses signal voltages appear at both terminals itl and In indicating a binary number 11 corresponding to the number 3.

het it now be assumed that after the counter has reached a count of three with current iiow in "on" tubes Il and 64 and increased output voltages appearing at both terminals ISI and Ill, the potential between control leads |26 and I is reversed in polarity, as would be the case if the Additional stages of the counter, as for exwhich the current iiow is through the o tube. P

Upon the application of a fourth impulse at terminal 12. however, counter stage M returns to the imoperated condition removing the output voltage from terminal I and at the same time applying a negative impulse to counter stage f ll. This impulse serves to switch counter stage from the "on" condition of stability which it occupies at the conclusion of the third impulse to the "oif" condition removing the signal potential from terminal I" and at the same time applying a negative impulse to the third counter stage.A This negative impulse switches the now of ciment from the oil tube of this stage to the o tube and the increase in late potential of the oi!" tube produced by this action may appear at an output terminal connected to the plate of the of' tube as an output signal indicative of the operation of this stage.

The operation of three counter stages in counting four impulses may be summarized in a table as below:

i l Second Counter Third Counter oi! on on on oil complex wave amplitude were to decrease. As a result of such reversal of control potential, interstage tube |08 is carried well below cut-off while the bias on interstage tube I2 is increased until that tube is just below cut-olf. If such ad' justments in the operation of the interstage tubes are made by means which will be considered hereinafter, the counter comprising counting stages Il. Il and 5l may be caused to reverse and count' on" tube 60 to oil tube 62 placing this stage in its ofi condition with the result that an increase in potential appears at output terminal I. This switch in current flow results in an increase in the plate potential' of on tube 60 and a decrease in plate potential of "oiT tube 62. Since interstage tube |08 is far below cutoil, the increase of plate voltage of on tube wil be of no effect. Also, since tube ||2 is still cut of! though now operating just below cut-oil', the decrease o! plate voltage of off tube O2 will not cause any current flow in tube ||2. Consequently, the potential at output terminal |38 will remain high while that at terminal |30 is reduced to its lower value and the binary number on these terminals will be 10, corresponding to the count of two.

Upon application or another negative impulse at terminal 12 (the second impulse after reversal of the control potential), current flow in counter stage 8l is against switched, it being transferred from off tube 62 to on" tube t0. The resultant decrease in the plate voltage of on tube t2 has no further eilect since interstage tube |00 is cut oif. The increase in plate voltage of oif" tube l2, however, is sufilcient to cause current i'low in lnterstage tube ||2 with a. resultant drop in the plate voltage thereof. Accordingly, a negative impulse is applied through capacitors ||1 and Il! to the grids of tubes 6I and 6I in counter stage 56. Thus current flow for counter stage Il, which was in on tube 64 at the counts of two and three, is shifted to off tube It with a resultant decrease in output potential at terminal |31. The binary number on output terminals |28 and |36 is then made 01 corresponding to a count of one.

Prom the above it will be seen that the direction of counting may be reversed and the counter caused to count downwardly by reversing the potential on control leads |26 and |34. While the above consideration has been coniined to'the two counting stages 5l and 56, it will be apparent that a counter with any desired number of stages may be caused to operate in the same fashion through employment oi interstage circuits similar to that shown between stages 5I and 5I, between each pair of stages.

In the above it has been assumed that means were available for turning the counter on and l the stages.

oi and for controlling the direction of counting by adjusting the potential between control leads |26 and |34. Comparator I8 performs these functions by comparing a voltage proportional to the instantaneous amplitude of the complex wave (plus the bias from battery 5|) applied to inputv terminal 36 (Fig. 2) with a potential which has been defined above as being proportional to the number of impulses indicated as counted by the binary number appearing on the output terminals of the counter. This potential is derived from the counter in the following manner.

Plate-grid resistors 92, |02 and |42 of counterv stages 54, 56 and 58 respectively are tapped and the taps are connected through rectiers |44, |46 and |48 respectively and series resistors |50, |52 and |54 respectively to the grid of vacuum tube 40 which forms a part of comparator |8.

The locations of the taps on resistors 92, |02 and |42 are so chosen that when the respective counter stages are in the off condition with current owing in the off tubes, the potentials derived therefrom are slightly below ground potential, the negative potential furnished by bias battery 90 being sufllcient to overcome the relatively low positive potentials at the plates of the off" tubes during the time these tubes are conducting. Under these conditions rectiers |44, |46 and |48 eifectively prevent any potential from appearing across resistor 52 and being applied to the control grid of triode 40.

If for example, counting stage 54 is switched to the "on condition, the plate potential of off tube 62 and consequently the voltage at the tap of resistor 92 will rise and current will flow through rectifier |44, resistor |50 and resistor 52. Resistor |50 is so proportioned that the drop across resistor 52 in response to such current flow is proportional to the amplitude portion represented by operation oi' counting stage 54. Thus by way of example, resistor |50 may have such an ohmic value that 0.1 volt is developed across resistor 52 when countingstage 54 lis switched to its on condition denoting a count of one.

AThe circuits including rectiiler |46-resistor |52 and rectifier Mil-resistor |54 operate inthe same manner when the stages with which they are associated are switched to the on condition of stability. In the case of stage 56, however. resistor |52 is so proportioned that theflow of current through resistor 52 occasioned by operation of counting stage 56 is proportional to the amplitude increment of 2 represented by operation of that stage. If the potential drop across resistor 52 in response to the operation of counter stage 54 is made 0.1 volt as suggested by way of example above, the drop across this resistor t 12 rent ilowing in that resistor from the counting stage representing the smallest increment beyond an amount which would interfere with the operation o1' the system with the delity required for a particular application.

From the above it will appearthat the voltage developed across resistor 52 and applied to the control grid of triode 40 is proportional to the amplitude of the counting sum appearing as a binary number on counter output terminals |36, |38 |40.

It will be recalled that battery 5| in series with potentiometer 50'in the grid circuit of comparator tube 38 is effective to add a steady bias voltage to the complex wave applied at terminal 36. Conveniently the bias voltage so added is positive and equal to one-half the peak-to-peak amplitude of the largest complex wave to be accepted by the coder. Thus all complex wave amplitudes applied to the grid of comparator tube 38 will be positive and the signal applied thereto for zero complex Wave input at terminal 36 will be substantially one-half the total range acceptable by the coder.

If, for example, the maximum peak-to-peak amplitude is A, the bias voltage from battery 5| may be a which is equal to 1/2 A. Then, measured in respect to ground, the complex wave as applied to the grid of comparator tube 38 will have a range of values extending from 0 to 2a and vall such valuesv will be positive in respect to'ground. If potentiometer 50 in the grid circuit of comparator tube 38 is so adjusted that the voltage applied to the grid of this tube for a given instantaneous complex wave amplitude,

measured with the superimposed bias, is equal appearing on terminals |36 through |40 of the impulse counter. Such comparison is made by a tubes sa and 1o, the plate potentials of which vary respectively and inverselyas the complex wave amplitude applied through potentiometer 50 and measured as set forth above and the counting sum amplitude applied through resistor lu 52. 'Thus if the amplitude of the complex wave produced by operation of counter stage 56 mustr then be 0.2 volt. Similarly, the drop across resistor 52 produced by operation of the nth counting stage 58 must be proportional to the increment of `amplitude represented by operation of that stage or 0.1 2n volts. voltages developed across resistor 52 upon the operationof additional stages must. of course, be proportional in the same way to the amplitudes represented by operation of It will be understood that the voltages developed across resistor 52 by the circuits considered .above need not b e respectively 0.1. 0.2 0.1X2nvolts, but may have any values in the same proportion so long as the current flowing in resistor 52 in response to operation of the counting stage representing the largest at any instant exceeds that indicated by the counter, the plate potential of tube 38 will bey less than that of tube 40 and a potential difference will exist between leads |56 and |58 connected respectively to the taps of resistors |51 and |59 which are connected between the plates of tubes 38 and 40, respectively and thenegatlve terminal of bias battery |24. Similarly a potential diierence of the opposite polarity will occur when the amplitude of the complex wave is less than that of the counting sum output of the counter. These differences in potential are employed to turn the counter on whenever the two values are unequal and to determine the direction' in which the counter is to operate or count.

In the foregoing it has been assumed that negative impulses have been applied to terminal 12 of the counting circuit without reference to the source of these impulses. In the illustrative system of Fig. 2 such impulses are derived fromV an oscillator |60, the frequency of which must be sumcient to permit operation of the counter over its entire counting range with enough speed to permit accurate representation oi' all freamplitude increment does not disturb the cur- I4 Quncy GOmPOIlentS 0f the 001111316X Walle form 13 desiredtobetransmitted. llorexamplaoscillator |88 which may be of any conventional type may have a frequency of 500,000 or 600,000 cycles per second. Its output is coupled through tramionner |82 to a pair of control tubes |88 and |88 shown herein as tetrod. The secondary winiping of the transformer is center tapped and thej'push-pull signal developed in this winding a atpueu to the screengnds of the two control tubes. The cathodes oi these two tubes are connected together and to ground while the plates ane connected through a common load resistor |88 to the positive terminal o! battery |18, the negative terminal of which is grounded. Resistors |81 and |88 connected to the plates o! comparator tubes 38 and 88. respectively are tapped to provide suitable potentials for application to the control grids oi' counter control tubes |88 and |88.

Ii' the amplitude o! the counter sum output is equal to the instantaneous amplitude of the complexwavetobetransmitted,asmeasuredirom ground after the addition of the bias voltage from battery Il, no potential dinerence appears between comparator output leads |88 and |88 and the control grids of tubes |88 and |88 are held at equal potentials Accordingly. the oscillator signal applied in push-pull to the screen grids of these tubes is canceled out in the plate circuit and no alternating current output is produced. Ifontheotherhand,apotential diiierence of either polarity occurs between the output leads from comparator tubes 88 and 88 in response to a disparity between the instantaneous amplitude of the complex wave to be transmitted. measured as above, and .the amplitude of the sum output of the coimter. the current through one of controltubesland I88willexceedthatthrough the other and an alternating current component will appear across plate resistor |88.

The potential developed across plate resistor |88 of counter control tubes |88 and |88 is appliedtothescreengridolaswitchingorgating tube |12 having at least two grids. The cathode of this tube. which is shown as a tetrode, is grounded and the plate is connected through load resistor |18 to the positive terminal of battery |18, the negative terminal of which is grounded. It will be assumed for the purpose of the present consideration that the potential applied to the control gird of switching tube |12 is such as to permit conduction of current therethrough whenever the voltages applied to the plate and screen grid have suitable values. It will be noted that the screen grid of switching tube |12 is connected directly to the plates of counter control tubes |84 and |88. Accordingly, a positive voltage of varying amplitude is applied to the screen grid of switching tube |12. This voltage is so chosen in relation to the other voltages applied to tube |12 (by variation of the potential oi' battery |18 or load resistor |88) that this tube is rendered conductive only when the sinusoidal plate signal across resistor |88 reaches a positive peak and the voltage on its control grid is simultaneously great enough to permit the flow of current,l

Thus, if the voltage applied to the control grid of switching tube |12 is suitable and if, because the sum output of the counter is not representative of theinstantaneous amplitude of the complex wave to be transmitted, a sinusoidal voltage appears across load resistor |88, switching tube |12 will pass plate current for each positive peak of the applied sinusoidal voltage. Accordingly, a series of negative pulses are developed across resistor |18 whenever the counter output. is not representative oi" the complex wave amplitude. These negative pulses are applied at terminal 12 to the iirst stage o! the counter, resistors 88 and 82 and capacitors 18 and 18, respectively being proportioned to differentiate and thus sharpen the negative pulses applied to counting' tubes 88 and 82.

'I'he circuits thus i'ar described illustrate the manner in which the counter is turned on or oi! depending upon whether or not the sum indicated thereby is representative of the amplitude of the complex wave to be transmitted. Means for controlling the direction o! counting of the counting circuit will now be considered. For this Purpose the potential dinerence existing between comparator output leads |88 and |58 is amplilled and applied to control leads |28 and l" to control interstage tubes |88, II2. etc. In Fig. 2 the ampliiier is shown as comprising vacuum tubes |18 and |88 but a multistage amplifier may be employed, the principal requirement being that when the counter sum differs from the complex wave amplitude by an amount equal to a count oi' one unit. the voltage developed between control leads |28 and |82 must be suiiicient to apply the proper bias to the interstage tubes as considered above. The cathodes of tubes |18 and |80 of the amplifier shown in Fig. 2 are connected together and to ground while the plates thereof are connected through load resistors |82 and |88 respectively to the positive terminal oi' tapped battery |88, the negative terminal of which is connected to ground. The screen grids of these tubes |18, |80 are connected together and to the tap of battery |88 while the control grids are connected respectively to the taps on comparator output resistors |88 and |81. In Fig. 2 the grids of ampliiier tubes |18 and |88 have been shown as connected to the same taps on resistors |58 and |81 as the grids oi' control tubes |88 and |84. It will be understood, of course, that these two sets ot connections need not be made to the same taps and may be varied as desired to obtain the proper potentials i'oi application to the grids of the control and amplitying tubes.

In considering the operation of the counter directional control circuit, let it be assumed that the counting sum voltage appearing across resistor 82 is less than the instantaneous amplitude of the complex wave to vbe transmitted. measured with respect to ground and applied to the grid oi' comparator tube 88. Under these conditions the plate current in tube 38 exceeds that in tube 88 and as a result, the plate voltage of tube 88 drops below that of tube 80. Accordingly, the voltage applied to the grid of ampliiler tube |88 is less than that applied to ampliiier tube |18. As a result, the plate current in amplifier tube |88 is less than that in amplifier tube |18 and the plate potential of amplifier tube |88, and thus the potential on control lead |26, is higher than the plate potential of amplifier tube |18 and thus the potential on control lead |34. Then the interstage tubes |88, |88, etc. are biased just below cut-oi! and can pass positive pulses from the on counter tubes, while interstage tubes I2, |88, etc. are biased far below cut-ot! and effectively open the interstage circuits between the off tubes of the counter. Thus the counter will count upwardly as has been described above, tending to increase the counting sum and thus to balance the comparator control circuit.

In a similar fashion, the counting direction of the counter is reversed whenever the sum output voltage appearing across resistor 52 exceeds the complex wave amplitude as measured from ground after imposition of the bias from battery 5|. Because of the bias from battery 5| the counter will rest at,the approximate middle of .its range whenever the complex wave applied at terminal 36 is of zero amplitude. As the complex wave at this point becomes more positive, the counter will count upwardly and as the applied complex wave becomes negative (at terminal 36) the counter will reverse and count downwardly.

From a consideration of the circuit arrangements described above, it will be seen that the potentials appearing at counter terminals |40 |38 and |36, taken in the order given, form a binary code group representative of the instantaneous amplitude of the complex wave applied at terminal 36 from terminal equipment 2. This binary code group changes continuously so that it is maintained at all times representative of the instantaneous amplitude of the complex wave as that amplitude increases and decreases.

The potentials at terminals |40 |38 and |36 may be transmitted to a remote station by a variety of means. For example, individual transmission subchannels each comprising a separate wire transmission line might be pro- `vided or known methods of multiplex may be employed. Conveniently and as shown in the embodiment illustrated in Fig. 2 means are provided for transmitting the code group formed by these output potentials over a radio transmission link comprising a separate subchannel for each of the counter stage output voltages. Because of therequirements of the particular receiver with which the exemplary system described herein is designed to operate, means are provided to render this radio transmission link operative only at predetermined times, during which times the coder is momentariy stopped to permit transmission of a fixed binary code group to the receiving station. In other cases it may be desirable, depending upon the receiver employed, to operate both the coder and transmitter continuously or to operate the receiver periodically interrupting the coder during the transmitting intervals as in the exemplary system.

The radio transmitter of the exemplary system comprises power amplifier tubes |90, |92 and |94, which may be of the vpentode type, associated respectively with counter stages 54, 56 and E8. The cathodes of power amplifier tubes |90, |92 and |94 are connected together and to ground while their plates are respectively connected through transformers |96, |98 and 200 to a dipole antenna 202, plate potential for the tubes being applied from battery 204 through the primary windings of these transformers. Associated with the power amplifier tubes |90, |92 and |94 are radio frequency oscillators 206, 208 and 2|0, respectively. If additional counting stages are employed, additional amplifiers and oscillators must be provided, one oscillator and one amplifier being associated with each counting stage as shown in Fig. 2. The oscilators are tuned to different radio frequencies and their outputs are applied to the suppressor grids of the associated amplifier tubes superimposed upon a bias voltage derived from battery 2|2. The counter output voltages from counter terminals |36, |38 and |40 are applied respectively to the screen grids of amplifier tubes |90, |92 and |94. If a suitable potential is vapplied to the control grids of amplifier tu'zes |90, |92 and |94, the radio frequency signals generated by the oscillators, respectively associated with theseA amplifier tubes will be applied to the antenna whenever the counter stage associated with a particular amplifier tube is in its on condition. When a counter stage is in its o condition, the potential appearing at its output and applied to the screen grid of the associated amplifier tube is insufficient to permit conduction.

The radio transmitter is turned on and the operation of the counter interrupted momentarily by means of a timing circuit to permit the transf mission of a code group representation of the instantaneous amplitude of the complex wave. In the exemplary embodiment described and illustrated herein the transmitting process is made to occur periodically under the control of an oscillator 2|2 which may be any conventional oscillator capable of producing a sinusoidal output signal of relatively constant frequency. If aperiodic transmission is desired a device capable of producing impulses at random intervals may be substituted for the oscillator. One such device may include means for amplifying a random noise potential and clipping it to produce a series of irregularly spaced impulses suitable for the operation of the timer.

In the present embodiment in which transmissions occur at regular intervals oscillator 2|2 may conveniently have an operating frequency of approximately 8,000 cycles per second. At this frequency at least two code groups will be transmitted for all frequency components of the complex wave to be transmitted having frequencies equal to or less than 4,000 cycles per second. If it is desired to transmit higher frequency components the oscillator frequency must necessarily be increased, and this may be done without substantial llimitation so long as the counter is capable of sufliciently rapid operation to permit counting of the ful amplitude range between the times of transmission. Since the counter operates reversibly 'and continuously measures the instantaneous amplitude of a complex wave to be transmitted, it is not necessary to allow any time for resetting the counter. Accordingly, for a given counter speed, higher frequency components may be transmitted over the system of the invention than over those previously known. Returning now to the illustrative embodiment shown in Fig. 2,' the sine wave output of oscillator 2 I2 is supplied through series resistor 2|4 to the control grid of a vacuum tube 2 |6. The cathode of this tube, which may conveniently' but not necessarily be a triode, is grounded while the plate is connected'through load resistor 2|8 to the positive terminal of battery 220, the nega'- tive terminal of which is connected to ground. Series resistor 2|4 in the grid circuit of this tube tends to flatten the sine wave and produces a signal across load resistor 2|8 having a wave form of the type shown at 222. This signal is applied to a circuit comprising resistor 226 and capacitor 224 which are of such values that the signal is differentiated, producing a wave form of the type shown at 228. The positive portions of vthis differentiated signal are eliminated by rectifier 230 through which the signal is impressed upon resistor 232 and capacitor 234 connected in parallel. The negative portions produce charges on capacitor 234 that are discharged re'atively slowly through resistor 232. The resultant negative saw-tooth pulses 236 are impressed through bias battery 238 upon the control grld of switching tube |12. Bias battery 236 is of a value to permit current iiow through switching tube |12 and each ot the negative peaks oi' the signal at 236 serves to cut oft the flow of current through this tube |12 to interrupt the operation of the counter. The values of capacitor 234 and resistor 232 are so chosen that switching tube |12 is cut on' for only a small part of the operating cycle of the timer.

The voltage appearing across resistor 232 and capacitor 234 in parallel is diilerentiated in the R. C. combination comprising capacitor 246 and resistor 242 to produce a signal having a wave form such as that shown at 244, the signal having a relatively large negative peak and a small positive peak. This signal together with the negative bias voltage supplied by battery 243 is impressed upon the control grid of vacuum tube 246, the grid circuit of which includes resistor 242' connected in series with battery 246 to ground. This battery furnishes suillcient bias to cut oil this tube in the absence of applied signals. The cathode of this tube is grounded and the plate circuit includes load resistor 256 connected between the plate and the positive terminal of battery 252, the negative terminal c! which is connected to ground.

The duration of the transmitting period is determined by the capacitance of capacitor 246, the greater this capacitance. the longer the transmitting period. Referring w the wave forms shown in Fig. 2 it will be recognized that for each negative saw-tooth pulse of the wave shown at 236 a diierentiated signal, such as that shown at 244, will be produced. The entire wave form 244 occurs during the period oi each saw-tooth 236 when the operation of the counter is interrupted. The small positive pulse of wave 244 is employed to enable the transmitting equipment during some portion of this interval. the exact shape of the pulse being dependent upon the value of capacitor 240.

The positive pulse applied to the grid of tube 246 appears as an amplified negative pulse across load resistor 256 and this amplified pulse is applied through capacitor 263 to amplifier tube 254, the wave form of the voltage applied thereto being substantially as shown at 256. The negative pulse, however, has no effect on the output of tube 246 which is normally biased at cut-oil. The grid of amplifier 254 is connected through resistor 256 to the negative terminal of bias battery 266, the positive terminal of which is grounded while the cathode is connected to ground. The plate of the tube is connected through resistor 262 to the positive terminal of battery 264, the negative terminal of which is grounded. Ampliiler 254 serves to reverse the phase of the impressed signal and increase its level to a suitable value without materially altering its wave form. The positive pulse produced in the plate circuit of triode 264 is applied through capacitor 256 to the control grids of power amplier tubes |66, |62 and |64, these control grids being connected in parallel through resistor 266 to the negative terminal of bias battery 216, the positive terminal of which is grounded. The positive pulse so applied to the respective control g'rids of the power amplifier tubes permits current to ilow therethrough during a part of the time that the operation o! the counter is interrupted. i! positive potentials from the counting stages associated with the ampliiler tubes are applied to the screen grids. Thus radio frequency current can ow in power amplifier tubes |96, |62 or |64 only when a positive pulse from the timer occurs simultaneously with the application o! a positlve potential from the counting stages with which they are respectively associated.

` In the operation of the coder and transmitter shown by way of example in Fig. 2 the complex wave form to be transmitted is applied through terminal equipment to comparator I6 in which its instantaneous amplitude measured from ground with a superimposed bias sulcient to make all amplitudes of one polarity is compared with a voltage proportional in amplitude to the counting sum appearing at the counter output terminals |46 |36, and |36. If these two amplitudes diil'er, a difference o! potential is generated between comparator output leads |56 and |56 and this difference o! potential is applied to control tubes |64 and |66 to permit application oi! the output of counter oscillator |66 to the input of the counting circuit.

At the same time the polarity ot the dinerence in potential between leads |56 and |56 determines the direction in which the counting circuit operates. 'I'he phase relationships throughout the counter control circuits are such that the counter will count to increase its sum output it the instantaneous amplitude of the complex wave to be transmitted exceeds the amplitude of voltage proportional to the sum output generated by the counter and counts downwardly it the opposite condition obtains. i

In the presence of an amplitude difference, impulses i'rom oscillator |66 after passage through control tubes |64 and |66 are applied to switchlng tube |12 through which they pass to counter input terminal 12 unless transmission of a code group is in progress. In this latter event, switching tube |12 is cut oil and operation of the counter is momentarily interrupted for the duration of the transmission. At any time the groupot voltages appearing 'at the several counter voutput terminals |46 |36, and |36, respectively, of counter stages 56 56. and 54, taken in the order given, form a binary code representation of the number of impulses delivered by oscillator |66 and thus of the instantaneous amplitude of the complex wave to be transmitted. These voltages are applied to the separate subchannels of the radio transmitter which is switched on and oi by the timer which serves also to interrupt the operation of the counting circuit at predetermined instants and synchronizes the operation of the two units. In those subchannels of the transmitterin which the transmitting period coincides with the presence of an on signal voltage at the output of the associated counting stage radio frequency energy is transmitted.

The receiving equipment indicated generally at 26, 36 and 32 in Fig. 1 may be of any suitable type, for example, radio receiver 36 may be a conventional superheterodyne receiver capable of broad band response, the intermediate frequency output of which will include a diilerent frequency component for each oi' the activated subchannels. The intermediate frequency output of this receiver is applied to decoder 36 which is arranged to derive therefrom a complex wave form reproducing that impressed upon the input o! the transmitting equipment. One form of decoder suitable for this purpose is disclosed in Fig. 5 of United States patent 2,272,072, issued February 3. 1942 on an application led in the name of Alec Harley Reeves. Brieiiy, the decoder disclosed in this patent includes a group of illters for separating the several alternating current voltages present in the intermediate frequency output oi the receiver and applying them to'a counter having the same number of stages as the counter employed in the transmitter. (In the patent iive counting stages are employed.) These voltages are rectiiied and utilized to condition the counter so that the sum indicated thereon is the sa-me as that indicated at the output terminals of the transmitter. The rectified output of the receiver is also utilized to trigger on an impulse generating circuit which after a slight delay causes the counter to count until its full capacity is reached. During the counting period current is permitted to iiow through an ampliiler, the duration of this current ilow being proportional therefore to the difference between the total capacity of the counter and the sum indicated by the transmitted binary code group of voltages. This output is filtered to obtain the desired reproduction of the complex wave form applied to the transmitter. For further details concerning this receiver and decoder reference is made to the above-identiiied patent.

What is claimed is: l

1. In a communication system for transmitting complex waves, a coder for expressing the instantaneous amplitude of a complex wave to be transmitted in n-digit binary code comprising a reversible binary counter of n stages each stage having an operated and an unoperated condition, output connections from each of said stages providing an output voltage change when the corresponding stage is in the operated condition, and means responsive to the amplitude of the complex wave to be transmitted to control the direction of counting of said counter for maintaining the n-digit binary code group formed by the voltages appearing at said output connections representative of said amplitude.

2. In a communication system for transmitting complex waves, a coder for expressing the amplitude of. a complex wave to be transmitted in binary code of n denominational orders comprising a binary counter of n stages each stage having an on and an oi! condition, means for deriving from each on stage a voltage proportional to the binary code amplitude represented thereby, and means responsive to the sum of these voltages and to the amplitude of the complex wave to be transmitted for controlling the operation of the counter.

3. In a communication system for transmitting complex waves, a coder for generating an n-digit binary code representative of the amplitude of a complex wave to be transmitted comprising an n-stage binary counter each stage having an operated and an unoperated condition, means for deriving from each operated stage a voltage proportional to the binary code amplitude represented thereby, and means for comparing the sum of said voltages with the amplitude of the complex wave to be transmitted and causing the counter to operate whenever said sum differs from the amplitude of the complex wave.

4. In a communication system for transmitting complex waves, an n-stage binary counter for expressing the amplitude of a complex wave in n-digit binary code each counter stage having operated and unoperated conditions and an output connection at which a voltage change occurs when that stage is operated, the voltages at the output connections forming an n-digit binary code group, means for generating a voltage proportional to the amplitude represented by said code group, and means responsive to the diiIerence between the amplitude of the complex wave 20 to be transmitted and the amplitude of said generated voltage tor causing the counter to operate to equalize said amplitudes.

' 5. In a communication system for transmitting complex waves, a coder for expressing the amplitude of a complex wave in n-digit binary code comprising a reversible n-stage binary counter each stage having operated and unoperated conditions and an output connection at which a voltage change occurs when that stage is operated. the voltages at the output connections forming an n-digit binary code group. means for generating a voltage proportional to the amplitude represented by said code group. and means responsive to the diiierence between the amplitude of the complex wave to be transmitted and the amplitude of said generatedvoltage for causing the counter to operate in the proper direction to equalize said amplitudes.

6. In a communication system for transmitting complex waves, a counter for expressing the amplitude of a complex wave in n-digit binary code comprising an n-stage binary impulse counting circuit, an output connection for each stage, means for generating an output voltage for each operated stage proportional to the binary code amplitude represented thereby, a pulse generator for operating said counter, and control means for said pulse generator responsive to the sum of said output voltages and to said complex wave rendering it effective to operate th'e counter whenever the amplitude represented in binary code by the counter output voltages diilers from the amplitude of the complex wave to be-transmitted. I

7. In a communication system for transmitting complex waves, a coder comprising an n-stage binary impulse counting circuit. an output connection for each stage, means for generating an output voltage proportional to the amplitude increment represented by each operated stage, said voltages forming a binary code group, a pulse generator for operating said counter, control means for said pulse generator responsive to the sum of the output voltages of said operated stages and to said complex wave for rendering said pulse generator effective to operate the counter whenever the amplitude represented in binary code by the counter output voltages differs 'from the amplitude of the complex wave to be transmitted, separate subchannels for transmitting said counter output voltages to a receiving station, and means ior disconnecting the pulse generator from the counter circuit at predetermined times and transmitting said output voltages over said subchannels.

8. The method of providing code signals representative of complex signals to be transmitted comprising continuously producing code groups in response to a complex signal to be transmitted, regenerating from said continuous code groups 1 a representation of the signal to be transmitted,

- voltage proportional to the amplitude represente'd thereby..means for comparing said derived voltage with the instantaneous amplitude o! saidA The following references are of record in the RAYMOND A. HEISING.

REFERENCES CITED file of this patent:

UNITED STATES PATENTS Number Name Date Reeves Feb. 3, 1942 Number 22 Name Date Stibitz Jan. 12, 1943 Massonneau Apr. 10, 1945 Roberts Aug. 14, 1945 Mumma June 18, 1946 Labin Aug. 20, 1946 Kinkead Mar. 25, 1947 Morton et al Apr. 8, 1947 Johnson Aug. 26, 1947 Pierce Mar. 16, 1948 Pierce Oct. 12, 1948 Schelleng Nov. 9, 1948 

